Atomistic Modeling of Ion Beam Induced Defects in Si: From Point Defects to Continuous Amorphous Layers.
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Atomistic Modeling of Ion Beam Induced Defects in Si: From Point Defects to Continuous Amorphous Layers. Lourdes Pelaz, Luis A. Marqués, Pedro López, Iván Santos, María Aboy, Juan Barbolla Departamento de Electricidad y Electrónica. Universidad de Valladolid E-47011 Valladolid, Spain ABSTRACT We present an atomistic model that describes the evolution of ion induced damage ranging from individual defects to continuous amorphous layers. The elementary units used to reproduce the defective zones are Si interstitials, vacancies and the IV pair, which is a local distortion of the Si lattice without any excess or deficit of atoms. More complex defect structures can be formed as these elementary units cluster. The amorphous pockets are treated as agglomerates of IV pairs, whose recrystallization rate depends on the local density of these defects. The local excess or deficit of atoms in the amorphous regions experiences some rearrangement as recrystallization takes place. In sub-amorphizing implants amorphous pockets are disconnected and when they recombine, they leave behind the local excess of Si interstitials and vacancies. When a continuous amorphous layer initially extends to the surface, the excess or deficit atoms within the amorphous layer are swept towards the surface where they are annihilated and only the defects beyond the amorphous-crystalline interface remain.
INTRODUCTION Ion implantation is the most common technique used to introduce dopants in Si. The passage of an energetic ion through a solid will initiate a sequence of displacements events that leads to defect production and, at sufficiently high doses, to a crystal-to-amorphous transformation of the irradiated silicon lattice. The formation of amorphous Si is beneficial because it not only limits ion channelling (which can distort the implanted dopant profile) but it is easily annealed at 500-600ºC [1]. Regrowth of the amorphous layer results in a low defect density in the recrystallized volume and the incorporation of the dopants into electrically active positions [2]. This behaviour differs from that of highly damaged but not continuous amorphous layers. Annealing temperatures of 800-1000ºC are required to remove extended defects and place the implanted ions into substitutional positions. The study of the evolution of defects resulting from ion implantation is relevant because they drive transient enhanced diffusion and clustering of dopants [3,4]. A significant experimental work in the field has allowed the characterization of these defects [3,5,6] and theoretical models have been developed to describe the observed defect evolution [7-9]. However, few models are able to deal with the damage accumulation of complex structures to form amorphous layers. In most cases, amorphization is treated in a very simplistic way assuming that the lattice turns amorphous if a critical point defect density is exceeded [10]. In this work, we present an atomistic model to describe a wide variety of damage generated by ion implantation, going from point defect
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